Method of reducing process plasma damage using optical spectroscopy

ABSTRACT

Optical emission spectra from a test wafer during a plasma process are measured using a spectrometer. The plasma charging voltage retained by (detected by) the test wafer is measured after the process step is completed. The emission spectra are correlated with the plasma charging voltage to identify the species contributing to the plasma charging voltage. The optical emission spectra are monitored in real time to optimize the plasma process to prevent plasma charging damage. The optical emission spectra are also monitored to control the plasma process drift.

CROSS-REFERENCE TO RELATED APPLICATION

This application takes priority under U.S.C. 119(e) of U.S. ProvisionalApplication No.: 60/384,499 filed May 30, 2002 entitled, “METHOD OFREDUCING PROCESS PLASMA DAMAGE USING OPTICAL SPECTROSCOPY” by Shiqun Gu,Peter McGrath, and Ryan Fujimoto, which is incorporated by reference inits entirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to processes for monitoring plasmacharging in semiconductor wafers. More particularly, the presentinvention relates to the use of optical emission spectroscopy in plasmaetching and deposition processes.

2. Description of the Related Art

Semiconductor wafer fabrication involves a series of processes used tocreate semiconductor devices and integrated circuits (ICs) in and on asemiconductor wafer surface. Fabrication typically involves the basicoperations of layering and patterning, together with others such asdoping, and heat treatments. Layering is an operation used to add thinlayers of material (typically insulator, semi-conductor or conductor) tothe surface of the semiconductor wafer. Patterning is an operation thatis used to remove specific portions of the top layer or layers on thewafer surface. Patterning is usually accomplished through the use ofphotolithography (also known as photomasking) to transfer thesemiconductor design to the wafer surface.

In the wafer fabrication, plasma enhanced chemical vapor (PECVD)deposition and plasma etching are used respectively to deposit thinfilms and etch patterned wafers. During these plasma processes, metallines (acting like an antenna structure) can collect charge from theplasma and transfer the charge to poly gates via metal interconnects. Ifthe plasma charge voltage is high, such as, for example, exceeding 6volts, the charge accumulated on the poly gates can damage the gateoxide. This results in a weak gate oxide and induces high leakage.Therefore continuous monitoring and reduction in plasma charging iscritical for device manufacturing.

Conventional approaches to the monitoring of plasma charging include theuse of wafer level reliability wafers (WLR) or EPROM wafers. In oneapproach, the wafer level reliability (WLR) wafers are processed throughthe production line. The WLR wafer has various types of metal antennastructures, which mimic those typically used in a manufacturing site.The wafers are tested upon fabrication completion, and the quality ofgate oxide can be tested by measuring the charge breakdown voltage(QBD). Alternatively, the plasma charge can be tested by the thresholdvoltage (V_(t)) shift. But testing using WLR wafers to identify plasmacharging damage is exceeding slow, sometimes taking several weeks ormonths to receive the data feedback. Moreover, the results depend on thequality of the gate oxide. Finally, results from individual plasmaprocess steps interact with other plasma process steps, making itdifficult to isolate the damage caused by a particular plasma operation.

Similarly, the EPROM wafer testing is a slow process, typically takingseveral days to yield useful data. EPROM test wafers EPROM wafers arealso very expensive, the cost further increasing due to the test wafer'ssusceptibility to plasma damage and the resulting need for frequentreplacement EPROM test wafers.

Accordingly, it is desirable to provide a more effective method andapparatus for monitoring and reducing plasma charge damage.

SUMMARY OF THE INVENTION

To achieve the foregoing, the present invention provides methods andapparatus for determining plasma charge voltages and correlating plasmadamage using optical emission spectra. The present invention provides animproved process for determining species contributing to plasma chargingdamage and using optical emission spectra for those species to optimizethe plasma process or to control the plasma process. The presentinvention also provides an apparatus to monitor process variations usingoptical emission spectra. Selected optical emission peaks, such asdetermined using optical emission spectrometry techniques, correlatewith plasma charging. These peaks may be measured in real time tooptimize the plasma process to avoid plasma processing damage.Alternatively, the emission peaks may be used to monitor the plasmaprocess drift to match a plasma process in one chamber with the sameprocess in a different chamber.

In one embodiment, the present invention provides a method fordetermining process species associated with plasma charging damage.Optical emission spectra from a test wafer are measured. The plasmacharging voltage detected by the test wafer is measured. The emissionspectra are correlated with the plasma charging voltage to identify thespecies contributing to the plasma charging voltage.

In another embodiment, the present invention provides a method foradjusting plasma process parameters by monitoring the optical emissionspectral output. A first optical emission spectrum correlated withplasma charging is measured. A plasma process parameter is adjusted inresponse to the measured first optical emission spectrum. In oneembodiment, the first optical emission spectrum is measured also afterthe adjustment of the process parameter.

These and other features and advantages of the present invention aredescribed below with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a plasma chamber optical spectroscopysystem to measure and analyze plasma emissions from a process chamber inaccordance with one embodiment of the present invention.

FIG. 2 is an example graphical representation of relative intensity as afunction of wavelength from a spectrometer in accordance with oneembodiment of the present invention.

FIG. 3 is a flowchart illustrating a method for determining a processspecies associated with plasma charging in accordance with oneembodiment of the present invention.

FIG. 4 is a flowchart illustrating a method for adjusting processparameters using optical spectral output in accordance with anotherembodiment of the present invention.

FIG. 5 is a flowchart illustrating a method for monitoring processvariations in accordance with another embodiment of the presentinvention

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made in detail to preferred embodiments of theinvention. Examples of the preferred embodiments are illustrated in theaccompanying drawings. While the invention will be described inconjunction with these preferred embodiments, it will be understood thatit is not intended to limit the invention to such preferred embodiments.On the contrary, it is intended to cover alternatives, modifications,and equivalents as may be included within the spirit and scope of theinvention as defined by the appended claims. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. The present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order not to unnecessarily obscure the present invention.

The present invention provides in one embodiment an improved process fordetermining species contributing to plasma charging damage and inanother embodiment an apparatus to monitor process variations usingoptical emission spectra. Selected optical emission peaks, such asdetermined using optical emission spectrometry techniques correlate withplasma charging. These peaks may be measured in real time to optimizethe plasma process to avoid plasma processing damage. Alternatively, theemission peaks may be used to monitor the plasma process drift or tomatch a plasma process in one chamber with the same process in adifferent chamber.

FIG. 1 is a diagram illustrating a plasma charge measuring apparatus tomeasure plasma emissions from a process chamber in accordance with oneembodiment of the present invention. The plasma charge measuringapparatus 100 includes a process chamber 106 for performing a plasmaprocess operation, such as plasma enhanced chemical vapor deposition(PECVD) or plasma etching. The plasma chamber or reactor may be anysuitable plasma chamber for performing plasma deposition or etching, asare known to those of skill in the art, such as including manycommercially available models. For example, the LAM XLE Pad Etcher,manufactured by Lam Research Corporation, Fremont, Calif.; the AppliedMaterials 5300 Centura etching chamber, manufactured by AppliedMaterials, Corp., Santa Clara, Calif.; and the ULVAC Asher, manufacturedby ULVAC Japan Ltd. are three examples of the many suitable plasmaprocess chambers for use with the present invention. Typically, theprocess chamber in the commercially available devices is constructedfrom stainless steel or aluminum materials, although other materials arebelieved to be suitable.

The wafer 102 is typically loaded and placed onto the bottom portion ofthe process chamber 106 and supported by a substrate holder.Commercially available plasma chambers range from manually loadedsystems, such as single wafer hand loaded systems, to automaticallyloaded systems. The plasma is typically generated by applying an RFpower through electrodes within the process chamber 106, one electrodeforming a cathode and another electrode forming an anode. When a gas issubjected to a DC or RF potential between the electrodes at reducedpressures, it produces a glow discharge and dissociates. This glow isdue to the electronically excited species producing optical emissions,the optical emissions characterized by the specific composition of theplasma discharge gas. The dissociated molecules more readily react withthe layers on the wafer, thus reducing the time required for processing.

The process gases 104 are typically introduced into the process chamber106 through a valved inlet line. The process gases 104, i.e., plasmafeed gases, vary according to the deposition or etch process stepsinvolved. For example, the process gases 104 may include any one or moreof the following gases: CHF₃, SF₆, O₂, HBr, Cl, NF₃, and CF₄. As afurther particular example, the fluorinated plasma process gases aretypically used for the etching or ashing of oxide layers. The recitationof the example process gases is intended to be illustrative and notlimiting. The scope of the present invention is intended to be extendedto each variety or combination of process gases as may be used in plasmaprocess chambers by those of skill in the relevant art.

The process chamber also includes a viewport 108, for spectroscopicanalysis of the optical emissions. Viewports are typically used to viewthe deposition or etch process occurring inside the process chamber 106.In specific embodiments of the invention, an optical coupling link 110couples the viewport to an optical spectrometer 112 for analysis of theoptical emissions. Methods of coupling viewports to spectrometers areknown in the industry. For example, an optical fiber cable may be usedto connect the viewport to the spectrometer.

The spectrometer 112 enables real time monitoring of the plasma processsteps by performing optical emission spectrometry. The opticalspectrometer functions to resolve the optical emissions received fromthe viewport into constituent wavelengths. Particular molecular speciesmay be uniquely characterized by one or more unique optical emissionspectral peaks. It is expected that the relevant information for theplasma processes and plasma charge monitoring will be contained in the200 to 900 nm wavelength range, although the invention is not solimited.

After dispersing the optical emissions from the plasma gases within theprocess chamber 106 to spectral components, the information is stored onan image sensor 114, typically comprising either a photodetector or CCDarray. Each of these stores data in different ways. For example, agrating with multiple slits may be used to control the projection of thespectrum of output light beams from the optical spectrometer 112 ontothe multiple elements of a photodiode sensor array, such as an array inthe image sensor 114. That is, the spectrometer splits the opticalemissions from the viewport into its full spectrum and the gratingenables the different spectral elements from the spectrum to beprojected onto different parts of the CCD array.

In an alternative embodiment, a single slit may be rotated inconjunction with a photodetector to record the intensities of theindividual frequencies included in the spectral content. Using either ofthese image sensing and storage methods, the spectral content may beacquired in a very short time period, typically less than one minute.

The optical emission spectrometry data acquired is then stored andanalyzed in data collection unit 116. Various embodiments of the methodof the present invention may implement the data collection unit, inwhole or in part, on a computing apparatus. Useful machines forperforming the operations of this invention include general purposedigital computers or other data processing devices. Such apparatus maybe specially constructed for the required purposes, such as for example,integrated circuits and microprocessors designed specifically to performthe function, or it may be a general purpose computer selectivelyactivated or reconfigured by a computer program stored in the computer.The data collection unit is configured, in accordance with oneembodiment, to determine a process species associated with plasmacharging, as illustrated in and described with reference to FIG. 3below. The data collection unit is configured, in accordance with oneembodiment, to adjust process parameters using optical emission spectraloutput as illustrated and described further below with reference to FIG.4. In yet another embodiment, the data collection unit is configured tofurther monitoring process variations by monitoring the spectral outputas illustrated and described further with reference to FIG. 5.

In order to correlate emission peaks to plasma charge or to performprocess optimization, test wafers are used to measure the plasma charge.As described earlier, wafer level reliability (WLR) wafers are used inconventional semiconductor processing to detect process changes thatcould adversely affect the product lifetime. This includes damage causedby plasma charging. Alternatively, EPROM wafers are used for these samepurposes. According to one embodiment of the present invention, thewafer 102 in two or more plasma process runs is either a WLR wafer or anEPROM test wafer and is used to develop a correlation database. That is,plasma damage charge as measured on a test wafer is correlated withoptical emission peaks as measured by an optical spectrometer 112.According to another embodiment, the process is optimized bymanipulating process parameters and observing the plasma chargingeffects (as determined using the optical emissions measured by theoptical spectrometer 112 and correlating the measurements to plasmacharging).

Since the use of optical emission spectrometry is non-invasive to theplasma processes and further provides real time data, variations in themonitored plasma process may be effectuated without the delaysassociated with conventional plasma charging such as through solereliance on test wafers, such as WLR wafers or EPROM wafers.

FIG. 2 is an example graphical representation of relative intensity as afunction of wavelength from a spectrometer in accordance with oneembodiment of the present invention. The optical emission spectrumgraphical representation 200 illustrates chemical species associatedwith increased plasma charge, as detected by a system such as thatillustrated in FIG. 1. For example, the free F emission spectral peaksidentified, i.e., peaks 206, 208, 210, 212, 214, and 216, correlate to ahigh plasma charge detected by a test wafer. Thus, a data collectionunit, such as illustrated and described with reference to FIG. 1, maycorrelate the intensities of such spectral peaks, according to oneembodiment, with plasma charge, as measured on the test wafer. While notwishing to be bound by a particular theory, it is believed that plasmacharge damage is a combination of RF and free F density in the plasma(when fluorine is used as a process species). The plasma charge voltage,for example as detected by test wafers manufactured by Wafer ChargingMonitors, Inc., Woodside, Calif., was observed to increase monotonicallywith the free F emission peak intensity detected by optical spectroscopyin the plasma process chamber 200. That is, the recipes which showedhigh plasma charge on WLR wafers also exhibited high free F emission onoptical spectra.

Thus, optical spectroscopy may be used to identify emission peakscorrelated to plasma charge. In order to measure the plasma charge, testwafers are used, as conventionally known in the relevant art. Whilefluorine species have been described as having OES peaks that correlateto plasma charging, the invention is not so limited. The invention isintended to extend to all process species having peaks which correlateto plasma charging.

FIG. 3 is a flowchart illustrating a method for determining a processspecies associated with plasma charging in accordance with oneembodiment of the present invention. The method for determining aprocess species associated with plasma charging begins at an operation302. Next, process species and process parameters are selected in anoperation 304. While not wishing to be bound by any theory, the amountof plasma charging is believed to be process dependent. That is, theplasma charging that may be measured, for example, on a test wafer, mayvary according to the levels of a variety of process parameters. Theseprocess parameters include, but are not limited to, pressures in theplasma chamber, process species flow rates, process species (i.e. thegases constituting the process gases), and RF levels. For example, andin accordance with one embodiment, the species and process parametersare selected in accordance with known general process recipes fordeposition or etching using specified process gases.

Next, in an operation 306, the plasma process steps are run on the testwafer. The test wafer may comprise any suitable test wafer, includingfor example, EPROM test wafers, such as CHARM wafers manufactured byWafer Charging Monitors, Inc. or WLR wafers which are processed in themanufacturing plant. In the CHARM wafers, metal antennas collect plasmacharge and store it in EPROM gates. The charge develops a voltage acrossthe gate and substrate capacitor, which changes the V_(t) of the EPROMtransistor.

In operation 308, optical emissions from the plasma are collected.Optical emissions are generated by the plasma and collected by thespectrometer concurrent with the running of the process steps inoperation 306. As noted above, each molecular species in the plasma maybe characterized by unique emission spectra. As described with referenceto FIG. 1, the optical emissions are collected in one embodiment by anoptical spectrometer optically linked to a viewport in the plasmaprocess chamber.

In a next operation 310, plasma charge is measured on the test waferafter completion of process operations. Techniques in determining theplasma charge detected by the test wafer vary with the wafers. Forexample, the EPROM test wafers collect the plasma charge and store it onthe EPROM gate, which shifts the threshold voltage (V_(t)). Aftermeasurement of the V_(t) shift, the plasma charge voltage and currentcan be detected. Since EPROM test wafers require the process steps to becompleted before measuring the threshold voltage, correlation of theplasma charging to optical emission spectrometry measurements are notmade in real time. For example, the plasma charge measurements may notbe available until a day after the commencement of the processing steps.Moreover, when other test wafers such as WLR wafers are used in thecorrelation setup process, plasma charge measurements may not beavailable for a week or longer.

If insufficient data is obtained to correlate plasma charging withoptical emission spectra, the process parameters are adjusted in anoperation 313 and operations 306-310 are repeated on a test wafer. Thatis, process steps are run on a test wafer, optical emissions collectedand plasma charging determined for the new process conditions. Theprocess parameters that may be adjusted in step 313 are extensive, andinclude, for example, any of the following: process species gas flow,chamber pressure, chamber geometry, RF power level, and processtemperatures. These process parameter examples are illustrative only andnot intended to be limiting.

After sufficient data for plasma charge correlation is obtained, theoptical emission data is correlated with the plasma charge in anoperation 314. This requires at least comparison of emission spectra attwo conditions, plasma charging and no charging, as detected by the WLRor EPROM test wafers. Comparison of the emission spectra at more than 2conditions will enable better correlation between the plasma chargingand the OES measurements. Correlation may be performed using suitablesoftware algorithms such as may be implemented on general or specialpurpose computers in ways known to those of skill in the relevant art,and will therefore not be described further here. For example, hardwareand software systems to facilitate correlating emission spectra arecommercially available, such as including one available from OceanOptics, Inc., of Dunedin, Fla.

With the correlation data as described, the intensity of the identifiedemission spectra may be measured and used to optimize the plasma processto avoid plasma charging damage or alternatively to monitor the plasmacharging in real time, as described further below with reference toFIGS. 4 and 5.

FIG. 4 is a flowchart illustrating a method for adjusting processparameters using optical spectral output in accordance with anotherembodiment of the present invention. The process begins in an operation402. Species and process parameters are selected in an operation 404.Next, in operation 406, process steps are run on a wafer and opticalemissions are collected. The wafer may be any production wafer. In oneembodiment, the wafer is a test wafer, such as for example, an EPROMwafer or a WLR wafer as described in greater detail above, to enableverification of the plasma charging. As described with reference to FIG.1, the optical emissions are collected in one embodiment by an opticalspectrometer optically linked to a viewport In the plasma processchamber. The plasma process parameters are adjusted, according to oneembodiment, to reduce plasma charging damage to the wafer, such as mayoccur to gate oxides.

Thereafter, a determination is made in an operation 408 as to whetherthe optical emissions, for example at the identified spectral peaks(selected frequencies), exceed a predetermined threshold. Thepredetermined threshold is determined from correlation data correlatingspectral peaks with plasma charging, such as developed and described bythe methods illustrated in FIG. 3. If the identified spectral peaksexceed the threshold, process parameters are adjusted in an operation410, and operations 406-408 are repeated until the plasma charging isreduced or the process is optimized. For example, for F basedchemistries, the intensities of free F peaks such as illustrated in FIG.2 at 691, 704, 713, 720, and 757 nm are monitored and the processparameters adjusted so that the emission intensities of free F isminimized. The method ends at an operation 416 with the processoptimized.

FIG. 5 is a flowchart illustrating a method for monitoring processvariations in accordance with another embodiment of the presentinvention. The process begins in an operation 502. In an operation 504,the species and process parameters are selected as in operation 404illustrated and described with reference to FIG. 4. Plasma process stepsare then run on the wafer and emission data is collected in an operation506, similar to that described with reference to operation 406 in FIG.4. Next, in an operation 508, the optical emission spectroscopic data isanalyzed. In one embodiment, the optical emissions are detected by anoptical spectrometer, sensed by a CCD array, and then stored in acoupled data collection unit, such as a general purpose computersuitably configured. The data collection unit identifies the emissionspeaks which are correlated to plasma charging and then uses theidentified peaks to monitor the plasma charging.

Correlation data previously accumulated, such as by using the methodsdescribed with reference to FIG. 3, enable the data collection unit todetermine the plasma charging levels in real time by monitoring opticalemission spectra. Next, in an operation 510, a resulting signal isgenerated, based on the evaluated optical emission measurements. Theresulting signal is preferably the intensity level as a function of timeof one or more wavelengths which have been correlated with plasmacharging.

The resulting signal is next compared in an operation 512 with apredetermined signal to determine to determine if the process hasdrifted, or in an alternative embodiment, whether the level from thechamber exceeds the levels from a baseline chamber. Preferably, if themeasurements are outside of a predetermined limit, the operator followsan out of control action procedure in a next operation 514 which in oneembodiment involves shutting down the process to make adjustments inorder to control the process drift. The out of control action planprocedures may include, for example, manual or automatic signals foradjustment of process parameters or alternatively may include advisorysignals or any appropriate signal based on the measure of divergence ofspectral values (i.e., the measured intensity levels) from predeterminedvalues. The procedures preferably include a shut down of the process tomake process adjustments or alternatively, in other embodiments, providefor adjustments of the process without shutting down.

If measurements are to continue as determined in operation 515,operations 504-514 are repeated until, for example, the process ends. Inan alternative embodiment, monitoring of the optical emission spectramay be used for chamber matching. Plasma process chambers vary ingeometries and in other structural aspects that may affect plasmacharging and other operational characteristics. In order to match theprocess in one chamber to another, the monitoring of the opticalemission spectra may be used in a similar manner as described above withreference to FIG. 5. In such a situation, the out of control planprocedures, as identified in operation 514, including either shuttingdown the process or performing adjustment to the process (withoutshutting down) to bring the resulting signal level from the secondchamber within specified limits, such as, for example, thosecorresponding to a baseline levels from a first chamber.

The processes presented herein are not inherently related to anyparticular computer or other apparatus. In particular, various generalpurpose machines may be used with programs written in accordance withthe teachings herein, or it may be more convenient to construct a morespecialized apparatus to perform the required method steps. The requiredstructure for a variety of these machines will appear from thedescription given above.

As noted earlier, the conventional plasma charging monitoring methodspresents several problems. Using WLR wafers or EPROM test wafers tomeasure the plasma charging is expensive and imposes delays between theprocess runs and the generation of measurements. With the methods andapparatus of the present invention as described above, real time datamay be provided to optimize the plasma process or alternatively forprocess monitoring or chamber matching. Moreover, monitoring opticalemission spectroscopic data is non-destructive, thereby reducing testwafer expenditures.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

What is claimed is:
 1. A method for determining process speciesassociated with plasma charging damage, the method comprising: measuringa first optical emission spectrum from a plasma directed to a test waferduring a plasma process operation; measuring a plasma induced chargelevel on the test wafer; correlating the optical emission spectrum withthe plasma induced charge level; and from the correlation, identifying amolecular species contributing to the plasma induced charge level. 2.The method recited in claim 1, further comprising: measuring a secondoptical emission spectrum from a wafer; and measuring a second plasmainduced charging level on a test wafer before correlating the emissionspectra with the measured plasma charging levels.
 3. The method recitedin claim 1, wherein the test wafer is an EPROM wafer.
 4. The methodrecited in claim 1, wherein the test wafer is a wafer reliability levelwafer.
 5. The method recited in claim 1, wherein identifying a speciescontributing to the plasma induced charge level comprises identifyingthe wavelengths of peaks in the optical emission spectra correlated withthe plasma induced charge level.
 6. A method for adjusting plasma chargelevels on a wafer during a plasma process step by monitoring the opticalemission spectral output, the method comprising: a) measuring a firstoptical emission spectrum correlated with plasma charging on a waferduring the a plasma process step; b) using correlation data to determinea plasma induced charge level on the wafer from the measured opticalspectrum; and c) adjusting a plasma process parameter in response to themeasured first optical emission spectrum when the plasma induced chargelevel exceeds a predetermined threshold.
 7. The method recited in claim6 wherein the optical emission spectrum is measured using an opticalspectrometer.
 8. The method recited in claim 6 wherein the adjusting theprocess parameters comprises one of adjusting the RF level, the pressurefor the species, and the flow rate for a species.
 9. The method recitedin claim 6 further comprising d) measuring the first optical emissionspectrum correlated with plasma charging from a wafer during a plasmaprocess step after the adjustment of the process parameter.
 10. Themethod recited in claim 9 further comprising repeating steps a through duntil the plasma charging is reduced.
 11. The method recited in claim 9further comprising repeating steps a through d until the plasma chargingis minimized.
 12. The method recited in claim 6 wherein the plasmaprocess parameter is adjusted to reduce plasma charging damage to thewafer.
 13. A method for monitoring plasma process variation, the methodcomprising: performing a first and second measurement of an opticalemission spectrum correlated with plasma charging from at least onewafer during a plasma process step; and determining the relative plasmacharge levels on the at least one wafer at the time of the measurementsfrom the the first and second measurements.
 14. The method recited inclaim 13 wherein the first measurement is taken in a different plasmachamber from the second measurement.
 15. The method recited in claim 14wherein the first and second measurements are taken to match conditionsin the chambers.
 16. The method recited in claim 13 wherein the firstmeasurement is taken in the same plasma chamber as the secondmeasurement but at a different time than the second measurement.
 17. Themethod recited in claim 13 further comprising, where the relative plasmacharge levels exceeds a predetermined threshold, adjusting a plasmaprocess parameter.